To minimise clogging, it would be necessary to do the pre-filtering if using the ultrafine syringe filters (and while there are larger systems available, it starts to get rather expensive).

It would be guesswork on my part if I was to attempt to say what the other 11% of that 89% material might be. Clay or other aluminosilicates would seem like the most likely candidates, and it could be another form of zeolite. Beyond that I'd have to look at the data and probably do a load of other background reading which, unfortunately, I can't prioritise right now!

I still wish I'd won that bid for the set of Endecotts sieves on ebay 10 years ago

Microplate Sieves for reliable particle analysis - Endecotts 5μm sieves!